Apparatuses of the vibratory or oscillating type are known, to convey a metal charge to a container of a melting plant, which container can be for example a melting furnace.
Such known apparatuses provide a segment long enough to pre-heat the metal charge sufficiently, by means of fumes exiting from the furnace, while it is being conveyed.
Each of the known apparatuses comprises a bearing structure on which a conveyor channel is mounted, having a substantially U-shaped cross section.
At least part of the conveyor channel is covered at the upper part by one or more hoods that define a tunnel into which the fumes exiting from the melting furnace flow, in the direction opposite to that in which the metal charge advances, and carry out the pre-heating.
These known apparatuses have the disadvantage, however, that despite the large quantity of fumes that are introduced into the tunnel covering the conveyor channel, at a relatively high temperature, in the range of 1300° C.-1400° C., only the upper layer of the metal charge, that is, the layer that is directly hit by the stream of fumes, is able to be adequately heated. The part below remains cold, or reasonably cold.
Therefore, a considerable fraction of the energy content of the fumes is not adequately used to heat the metal charge, and therefore the fumes leave the pre-heating tunnel at a temperature that is still rather high, and the metal charge exits from the conveyor channel at an average temperature of less than 100° C., even if the upper layer can reach higher temperatures.
Apparatuses are also known, for example from WO-A-94/09332, or from IT-B-1359081, in which in order to promote the distribution of the fumes and hence of the heat over the whole height of the metal charge, suction means are disposed on the lower wall or lateral walls of the conveyor channel, which take in a part of the fumes, obliging them to pass through the metal charge from the top to the bottom. In these known solutions, the fumes taken in then converge in the main discharge pipe.
However, in known solutions, the normal hoods that define the tunnel are disposed to cover the conveyor channel defining a space for the fumes to pass between the upper part of the mass of scrap and an internal surface thereof.
The passage space thus defined has a volume unrelated to the internal volume of the melting furnace and to the average quantity of fumes generated, so that the fumes transit at a high speed and remain for a short time in contact with the metal charge.
Even the solution that provides that the metal charge is hit by the fumes does not solve the problem, because it entails an excessive heating of the metal charge limited to the zone where it is introduced into the furnace; in fact, an excessive heating can cause localized melting in the charge, or possible explosions caused by the high temperatures and by the fact that the fumes are not completely combusted.
Localized melting, especially if it occurs in the zone where the charge is introduced into the melting furnace, can entail the formation of compact blocks of metal charge that cause blockages of the conveyor channel, with consequent long and complex maintenance interventions on the plant.
Another disadvantage of known solutions is that some gases present in the fumes, for example carbon monoxide (CO), remain unburned, and therefore need further processing before they are introduced into the atmosphere.
Furthermore, given the high speed of the fumes in transit in the tunnel, most of the particulate present in the fumes remains in the stream of fumes, so that the particulate and the suspended powders pass through the tunnel, requiring at exit specific filtering actions and possible recovery, with an increase in the costs and times in both producing, managing and maintaining the plant.
Moreover, the high temperature at which the fumes impact against the metal charge inside the tunnel causes oxidation of the scrap itself, requiring a greater energy consumption in order to melt them in the melting furnace; this also causes a loss of material, with a reduction in the yield of the metal charge.
One purpose of the present invention is to achieve an apparatus for conveying and, at the same time, pre-heating a metal charge in a melting plant, which has a high yield, that is, such that the heat energy given up by the fumes to the metal charge is as high as possible and, at the same time, the scrap can be heated uniformly, preventing, as much as possible, any oxidation thereof.
Another purpose of the present invention is to achieve an apparatus for conveying and pre-heating a metal charge which allows substantially both the complete combustion of the non-combusted gases and also, advantageously, the precipitation onto the metal charge of the particulate and powders present in the fumes arriving from the melting furnace.
Another purpose of the present invention is to achieve an apparatus that has limited management and maintenance costs compared with the state of the art.
Yet another purpose is to achieve an apparatus that has a limited environmental impact, in which the powders have already been largely filtered from the gases sent outside.